So I just upgraded our ES cluster to version 6.3. All nodes are up and running, as well as Kibana. Unfortunately our logstash instances refuse to connect to elasticsearch now, claiming that X-Pack Security needs to be enabled in Elasticsearch:

No, security is not required. I would start with removing the xpack.management.elasticsearch.password and xpack.monitoring.elasticsearch.password settings as these are not applicable without security enabled and see if that makes a difference.

Known issues
Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set xpack.security.enabled: true in elasticsearch.yml to upgrade successfully. If this value is not set, the cluster will be unable to form after upgrade.

Also see Breaking changes in 6.3.

Unfortunately nothing is noted in the breaking changes, or in regard to logstash having this requirement for 6.3. I tried with empty username and password with no avail. Guess we'll have to enable security...

It'd be helpful if this was indeed listed as a breaking change, or under the release notes for elasticsearch, or logstash. Neither of these places listed this issue!